Prime septuplets (p, p+2, p+6, p+8, p+12, p+18, p+20) are one of the two types of densest permissible constellations of 7 primes. Maximal gaps between septuplets of this type are listed in A201051; see more comments there.

The gap of 165690 between septuplets starting at p=11 and p=165701 is the very first gap, so a(1)=11. The gap of 903000 between septuplets starting at p=165701 and p=1068701 is a maximal gap - larger than any preceding gap; therefore a(2)=165701. The next gap of 10831800 starts at p=1068701 and is again a maximal gap, so a(3)=1068701. The next gap is smaller, so it does not contribute to the sequence.